As electronic devices are becoming cordless and more portable, it is desired to use non-aqueous electrolyte secondary batteries which have light weight and high energy density as the power source for electronic devices. In fact, the kind of electronic devices powered by non-aqueous electrolyte secondary batteries is significantly increasing. When non-aqueous electrolyte secondary batteries are used for commercial electronic devices, there is a problem to be solved. That is, non-aqueous electrolyte secondary batteries use lithium, which is highly reactive, as an active material, and therefore, in the event of a short-circuit, they produce greater amounts of heat than other commercially available batteries. That is, in non-aqueous electrolyte secondary batteries, when the positive electrode and the negative electrode come into direct contact with each other due to a short-circuit, heat is generated. Thus, the separator (resin porous film) separating the positive electrode from the negative electrode melts around the short-circuited area, so that the contact area of the positive electrode and the negative electrode increases. As a result, the short-circuited area expands and may be overheated. It is therefore common to employ a technique in which a porous heat-resistant layer composed mainly of a heat-resistant resin and an inorganic oxide is used together with the separator to prevent, in the event of a short-circuit, an overheat due to the expansion of the short-circuited area.
The porous heat-resistant layer is provided on the surface of an electrode such as positive electrode or negative electrode, and its thickness is adjusted to approximately 2 to 10 μm so as not to impair the design capacity of the battery. The preferable method for forming a very thin layer of such thickness is the gravure method. The gravure method is a method in which a paint that is a precursor of a porous heat-resistant layer is supplied to the outer surface of a gravure roll with a plurality of depressions, and the paint is transferred from the outer surface of the gravure roll to the surface of an electrode. The paint supplied to the outer surface of the gravure roll spreads over the outer surface of the gravure roll along the depressions. By making the transport direction of the electrode opposite to the rotation direction of the gravure roll, the thickness of the coating film applied to the electrode surface can be precisely controlled, and a thin coating film can be formed.
Also, there has been proposed a technique of providing a blade so as to contact the outer surface of a gravure roll (see, for example, Patent Documents 1 to 3). By providing such a blade, the excess amount of a paint spread over the outer surface of the gravure roll can be removed, so the amount of the paint on the outer surface of the gravure roll can be accurately adjusted. It is thus possible to more precisely control the thickness of the coating film applied to the electrode surface.
However, Patent Document 2 and Patent Document 3 use a metal blade. When a metal blade is brought into contact with a gravure roll for a long period of time, the wearing of the metal blade produces a metal powder, which may adhere to the outer surface of the gravure roll. The metal powder may enter the paint carried on the outer surface of the gravure roll, be transferred to an electrode surface, and finally adhere to, for example, the surface of the porous heat-resistant layer. By the way, an electrode with a porous heat-resistant layer formed thereon is laminated with another electrode of different polarity, with a separator made of a resin or the like interposed therebetween, to form an electrode assembly that is a power generating element. In such an electrode assembly, the adhesion of a metal powder to the porous heat-resistant layer is equal to the inclusion of a conductive foreign substance in the electrode assembly. This can cause a short-circuit, which is a phenomenon of electrical connection between two electrodes inside an electrode assembly due to partial destruction of a separator. Therefore, the techniques of Patent Document 2 and Patent Document 3 are not applicable to the formation of a porous heat-resistant layer on an electrode surface in the formation of a battery electrode.
From such a viewpoint, in utilizing the gravure method to produce a battery electrode, it is necessary to use an insulating material, such as resin, as the blade material so as not to cause a short-circuit even if the blade has worn away and its fragments enter the electrode assembly.
Patent Document 1 does not have a description of the blade material, so it can be construed as suggesting the use of a resin blade. However, with only the use of a resin blade, as the resin blade wears away, it becomes difficult to remove the excess amount of the paint carried on the outer surface of the gravure roll with good accuracy.
It is common to bring the tip of a resin blade into contact with the outer surface of a gravure roll such that the whole resin blade becomes slightly warped. In this case, the contact area of the tip of the resin blade and the outer surface of the gravure roll becomes small, and the excess amount of the paint can be continuously removed with good accuracy. However, as the tip of the resin blade wears away, the warpage of the resin blade decreases and the contact area of the tip of the resin blade and the outer surface of the gravure roll increases. As a result, a greater amount of the paint than the excess amount is removed, the amount of the paint applied to the electrode surface gradually decreases from the predetermined amount, and the thickness of the finally formed porous heat-resistant layer decreases. Since the porous heat-resistant layer is formed between electrodes, it is a factor that determines the distance between the electrodes, and the inter-electrode distance has a large impact on the battery performance of the finally obtained battery. Therefore, a decrease in the thickness of the porous heat-resistant layer inevitably results in variations in the battery performance.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-179151    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-202350    Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-276156